1. Field of the Invention
This invention relates to an improved process for preparing polychloronitrobenzene and more particularly to a process for synthesizing pentachloronitrobenzene directly from the corresponding pentachlorobenzonitrile compound.
2. Prior Art
Pentachloronitrobenzene is a well known fungicide as shown in Analytical Methods for Pesticides, Plant Growth Regulators, and Food Additives, Zweig, C., Ed., Vol. III, 127, Academic Press, N.Y. (1964). Heretofore, producing pentachloronitrobenzene required the use of the trichlorobenzene with a nitrating agent and excess chlorine in the presence of a strong chloro acid such as chlorosurfonic acid. Other methods of producing this compound include chlorinating nitrobenzene in the presence of a strong acid and iodine and nitrating pentachlorobenzene.
The prior art methods require the chlorination of the previously nitrated compound and/or the simultaneous nitration and/or chlorination of a partially substituted compound wherein the nitration occurs at a previously unsubstituted site. These prior art methods are somewhat costly. For example, the direct nitration of pentachlorobenzene is particularly disadvantageous owing to the difficulty and expense encountered in preparing the pentachlorobenzene starting material.
It has now been found that pentachlorobenzonitrile can be selectively converted to the corresponding pentachloronitrobenzene in accordance with the instant invention. This is surprising in that it is known that an acid anilide, such as acetanilide, (Ar-NHCOR), will not undergo hydrolysis and subsequent oxidation to the corresponding nitro compound. In this process the Ar-N bond is not broken but the ring is nitrated at the para position, Morrison and Boyd, Organic Chemistry 1962, p. 559. Additionally it is known that halogenated aromatics undergo nitration in the presence of nitric acid and a strong protonic acid by substitution of a halide group, Chemical Reviews Vol. 40, 1947. Therefore, reactions of the instant type led to polynitrated compounds formed by replacement of more than one chloro atom. Thus, it was particularly surprising when it was discovered that a fully substituted pentachlorobenzonitrile molecule containing Ar-CN bonding could be selectively converted to pentachloronitrobenzene. Such a process is particularly advantageous owing to the fact that an economical process for producing commercial amounts of pentahalobenzonitrile has recently been developed.